The present invention relates to an apparatus for counting particles in a liquid suspension and method therefor and more specifically to apparatus for counting both red blood cells and platelets in whole blood.
In a copending application titled "Particle-Density Measuring System" in the names of the present inventor and Bernard A. Shoor, and filed Nov. 4, 1976, Ser. No. 738,896 now abandoned, and continuation-in-part application Ser. No. 832,893, now U.S. Pat. No. 4,110,604 a technique is disclosed for determining the number of platelets per unit volume of a blood sample by the use of a ratio method. Specifically, red blood cells are counted simultaneously in the same sample with platelets to thus provide a ratio of platelets counted to red blood cells counted. Then if the red blood cell density is known, the platelet density is immediately available.
The general concept of electronically counting different types or sizes of particles has been suggested in a Coulter U.S. Pat. No. 2,656,508. However, because of the quantum difference in both size and volume between red blood cells and platelets there is great difficulty in obtaining an accurate platelet count. For example, as stated in an article entitled "Electrical Sizing and Counting of Platelets in Whole Blood" by J. Schulz and R. Thom, Medical and Biological Engineering, July 1973 pp. 447-454, on page 448
". . . it is difficult or even impossible to analyse a large and small particle population in the same suspension. This problem arises in many applications and is of special interest if platelets and red cells are to be analysed from whole blood. PA1 To overcome these disadvantages, the particles should be injected into the centre of the orifice only, and those particles outside the hole should be kept out of the current density field." PA1 To obtain stable conditions for the hydrodynamic focusing process and a constant diameter of the central particle-suspension stream, no pressure is applied to the jet capillary tube.
Fig. 1 of the Thom Article on page 447 shows the use of a jet capillary with the use of a front sheath for increased resolution. He further states on page 448 regarding the jet tube
A Thom U.S. Pat. No. 3,810,010 shows a capillary tube with the use of a front sheath apparently similar to the Thom article. The Thom article also cites on its page 454 a German Thom article dated 1972 entitled "Vergleichende Untersuchungen zur Elektronischen Zellvolumenanalyse" AEG Telefunken Publ. N1/EP/1698. The German Thom in FIG. 10 shows counting apparatus which has both front sheath and back sheath flows and apparently uses a jet capillary similar to the Medical and Biological Engineering Thom article. FIG. 10 shows that the pressure, P.sub.3, of the jet capillary is equivalent to the front sheath pressure, P.sub.2, which is much greater than the back sheath pressure, P.sub.1 which in turn is greater than the waste tube pressure, P.sub.0. In addition, the drawing appears to show that the pressure P.sub.1 is derived from P.sub.2 or P.sub.3 by a pressure reduction constriction.
An article by Spielman and Goren in the Journal of Colloid Interface Science 26, 175-182 (1968) entitled "Improving Resolution in Coulter Counting by Hydrodynamic Focusing" discloses the use of a front sheath for improved resolution.
Finally a cell sorter product manufactured and sold by Becton Dickinson Corporation under Model FACS, (the assignee of the present invention) uses a small bore tube as a fluid resistor connected in line with larger bore tubing for driving a sample fluid through an aperture which is at the apex of a conical structure into which the larger tubing extends. The cell sorter operates on the principle of dividing the fluid into droplets by means of the aperture and vibration and then sorting the droplets.